Which parasite is a large ciliate protozoan that may be found in swine feces?

• A. Trichomonas species
• B. Balantidium species
• C. Histomonas species
• D. Giardia species

Answer: (B)

Recognizing a large ciliate protozoan in swine feces points to Balantidium species. Balantidium is the only large organism among these options that is a ciliate, meaning it is covered with cilia all over its surface, which give it a distinctive, visible movement and structure under the microscope.

In the life cycle you’d expect to see two forms: a trophozoite that resides in the intestine and a cyst form that is shed in feces and is capable of surviving outside the host to initiate infection when ingested. The trophozoite is relatively large and features a prominent macronucleus (often described as kidney-shaped) along with a micronucleus, a hallmark of Balantidium’s ciliates. This parasite can be found in pigs and, through ingestion of cysts, can occasionally infect humans, making it a zoonotic concern.

The other organisms listed are all flagellates, not ciliates, so they don't fit the description of a large ciliate. Trichomonas and Giardia move by whip-like flagella and have different shapes and nuclei, while Histomonas meleagridis is a bird pathogen and also a flagellate, not associated with swine feces.